28 The Cephalic, Oral, and Esophageal Phases of the Integrated Response to a Meal
Upon completion of this chapter the student should be able to answer the following questions: 1.
What are the structures of the functional anatomy of salivary glands, including their secretory elements?
2.
What are the cephalic and oral phases (what, why, how it happens) of the response to a meal?
3.
What are the general principles of secretion along the gastrointestinal (GI) tract (where do secretions come from, what are the components)?
4.
How do the components of secretion vary with the gland or region of the GI tract?
5.
What is the correlation between the composition and functions of salivary secretion?
6.
How are primary and secondary secretion within salivary glands generated and regulated?
7.
What is the sequence of events in swallowing?
8.
What are the stimulus and neural pathways generating primary and secondary esophageal peristalsis?
9.
What changes in gastric motility take place during swallowing, and what is the significance?
10.
What are the major functions of the esophagus and associated structures in terms of protection and propulsion?
his chapter will describe the processes that occur in the gastrointestinal (GI) tract in the early stages of the integrated response to a meal. There are changes in GI tract physiology (1) before food is ingested (the cephalic phase), (2) when ingested food is in the mouth (the oral phase), and (3) when food is transferred from the mouth to the esophagus (the esophageal phase). The responses of the GI tract to the presence of food are mainly associated with preparing the GI tract for digestion and absorption.
The main feature of the cephalic phase is activation of the GI tract in readiness for the meal. The stimuli involved are cognitive and include anticipation or thinking about the consumption of food, olfactory input, visual input (seeing or smelling appetizing food when hungry), and auditory input. The latter may be an unexpected link but was clearly demonstrated in the classic conditioning experiments of Pavlov, in which he paired an auditory stimulus to the presentation of food to dogs; eventually the auditory stimulus alone could stimulate secretion. A real-life analogy is presumably being told that dinner is ready. All these stimuli result in an increase in excitatory parasympathetic neural outflow to the gut. Sensory input (e.g., smell) stimulates sensory nerves that activate parasympathetic outflow from the brainstem. Higher brain sites (e.g., limbic system, hypothalamus, cortex) are also involved in the cognitive components of this response. The response can be both positive and negative; thus anticipation of food and a person’s psychological status, such as anxiety, can alter the cognitive response to a meal. However, the final common pathway is activation of the dorsal motor nucleus in the brainstem, the region where the cell bodies of the vagal preganglionic neurons arise. Activation of the nucleus leads to increased activity in efferent fibers passing to the GI tract in the vagus nerve. In turn the efferent fibers activate the postganglionic motor neurons (referred to as motor because their activation results in change of function of an effector cell). Increased parasympathetic outflow enhances salivary secretion, gastric acid secretion, pancreatic enzyme secretion, gallbladder contraction, and relaxation of the sphincter of Oddi (the sphincter between the common bile duct and duodenum). All these responses enhance the ability of the GI tract to receive and digest the incoming food. The salivary response is mediated via the ninth cranial nerve; the remaining responses are mediated via the vagus nerve.
Many of the features of the oral phase are indistinguishable from the cephalic phase. The only difference is that food is in contact with the surface of the GI tract. Thus there are additional stimuli generated from the mouth, both mechanical and chemical (taste). However, many of